Preparation of shaped polyacrylonitrile structues

ABSTRACT

Polymers containing at least 80 per cent of acrylonitrile are dissolved in aqueous nitric acid of 46 to 68 per cent by weight concentration at temperatures below 80 DEG  C., and the solutions produced are processed at temperatures up to 180 DEG  C. into shaped structures such as films and fibres.  In a modification the acrylonitrile polymer is dissolved in a mixture containing 15 to 95 per cent of nitric acid and 85 to 5 per cent of nitromethane. Polymers of acrylonitrile specified are polyacrylonitrile and copolymers with vinyl pyrrole, vinyl chloride, vinyl acetate, styrene, butadiene, methacrylonitrile, and acrylic and methacrylic acids.  The process is preferably applied to polymers having a molecular weight between 10,000 and 90,000 which have been prepared with the assistance of organic catalysts.  The methods by which the solutions are prepared may be varied according to the temperatures and concentrations of nitric acid employed; when using a mixture of nitric acid and nitromethane the polymer is preferably first suspended in nitromethane and the suspension converted to a solution by the addition of nitric acid.  Reference has been directed by the Comptroller to Specification 701,652.

PREPARATION OF SHAPED POLYACRYLONI- TRILE STRUCTURES Paul Halbig,Fribonrg, Switzerland No Drawing. Application September 215, 1952 SerialNo. 311,530

Claims priority, application Switzerland September 29, 1951 8 Claims.(Cl. 18-54) solved in nitric acid and can be formed into articles,pargticularly filaments, as described in my copending patentapplication, Serial No. 236,081, filed July 10, 1951, now

v handoned, of which this application is a continuationdnp Therebychemical modifications of the dissolved "polymer may however easilyoccur, sothatthe shaped materials do not correspond any more totheoriginal comp sition and ep nd ng th g of the p cess d solution showdivergent properties, which is as a rule gfdisadvantage for industrialpurposes.

The object of this invention is a process for the .disselving ofpolymers of aCrylonit-rilei-n nitric acid and for the shap g of t s s in n p tern with t chemica composition of theinitial material. Thisobject is arrived at with the aid of definite measures which are givenin the following description.

The needed measures can be summed up in three groups: a

(1) The concentration of the acid, the way of preparingthe solution andthe temperatures during the operations are chosen in such a way that achemical change of the dissolved substance is prevented.

('2) The acid and the solution is freed of nitric oxides and stabilisedwith urea. 1

(3) The polymers are produced with definite catalysts.

The stability of the polyacrylonitri-le in "nitric acid is dependent toan extraordinarily great extent from the temperature. That wasto beexpected, but it is surprising that it is possible to dissolve thisnitrile in nitric acid and to bring it again to separation in anunchanged state. Definite polymers can be stored for several .days innitric acid solutions at 10-20 C. and then can :be regenerated onprecipitation substantially unaltered. A-t elevated temperature,especially over 80 C., rapid changes take place and at temperatures ofbetween 100 and 180 (3., these solutions remain stable hardly a fewminutes. The nitric acid containing solutions are therefore produced attemperatures under .80? C., preferably at 0e50 .C.', and stored, if thatis necessary, at 0-20" C. for the subsequent processing of thesolutions, higher temperatures may be used only for short periods oftime. At 80-90 C., Salt) minutes are admissible; at LOO-180 C., periodsshould not exceed two minutes, as long as the nitric acid is notcompletely removed. i I

As stated in the above mentioned patent application, nitric acid in theconcentration range from .4668% can she used. The dependence of thesolution from the :connntration and the temperature is illustrated inthe it'ollowingtable:

ice

46% 48% 50% 70 52% 5s 54% 2- 5 Under 15 Particularly suitable are acidswith a content of .52. a

65% so that temperatures above 55 C. are not necessary in thepreparation of the solution. Acids having a higher concentration than65% may already have a nitrating eliect on the polymer. For thestability of the solution the absence of nitric oxides is of furtherimportance. For this purpose, the solutions are prepared in vacuo,

whereby nitric oxides are sucked ofi as soonas they are formed. In thefinal solutions the formation of nitric oxides may be retarded by smallquantities of urea.

As stated in the patent application Serial No. 294,482, filed June 19,1952, now Patent No. 2,671,768, part of the nitric acid can be replacedbynitromethane, which of itself is not a solvent for polyacrylonitrile.The invention includes also the use of such nitric acid-nitromethanemixtures a solvent,

The nitric acid solutions of the polyacrylonitrile may be prepared invarious ways. The polymer may be agitated with an acid of'suitableconcentration, for instance with 60% nitric acid at 20 C. until it isdissolved, which requires several hours. A quicker procedure consists inmixing polymer powder at 0-20 C. first with an amount of 52 to 54%nitric acid suificient to obtain a liquid sus pension, which is thendegassed in vacuo and converted into a solution by heating. A stillbetter method, which vacuo. Subsequently, 65% nitric acid is added insuch an :amount as 'to produce with the starter nitric 'acido-f 52 to54% an acid having a concentration of 59%, whereby the suspension isconverted to a solution. Instead :of preparing the suspensions withdilute nitric acid as .described hereinabove, nitromethane may be usedforthis purposewhereuponthe suspension is converted into .a solution bythe addition of higher concentrated acid. The solutions canv also beprepared in continuous processes by conducting polymer and solventcurrently in the calculated'p-roportion to a mixing device, as forinstance a mixing worm. Thereby the production time can be shortened. Asuspension may first be produced from a 49-50% acid, degassed, and thissuspension then be converted in the screw-mixer by short heating at48-80 C. to a solution which is afterwards immediately spun. samesolution may also be mixed in the screw-mixer with a stronger nitricacid in such concentration and amount as to already produce a solutionat 10-20" C. and to then shape this solution.

Ihave also found that the stability of the polymerin nitric acidsolutions depends on the nature of the catalyst used forthe-polymerisation. The products prepared with persulfates andsulfur-containing activators are the most sensitive polymers insolution. Best suitable are the polym'ers prepared with organiccatalysts, of which I prefer the peroxides of aliphatic carboxylicacids. This was determined as follows:

A polyacrylonitrile having a molecular weight of 85,000

the letter B. Prom A and B, 8% solutions .in 58%1'1itfic acid wereprepared and stored at 20 C. In

Patented Mar. 15;,

V invention, parts are given by weight.

tervals, the viscosity of the two solutions was determined by theball-drop method (see Analytical Chemistry, vol.

22 (1950), page 656). I obtained the following ball-drop It will benoted that the viscosity of the solution A (persul-fate) decreasesquickly and continuously, whereas I the viscosity of the solution B(peroxide) remains,.after' an initial increase, substantially constant.

Similarly, the superiority of the solution B is also 'shown by thedetermination of the weight increase of the dissolved polymer, whichincrease is due to a partial saponification of the nitrile group. 1found the follow- "ing increase for the same polymers and the samesolutions prepared under the same conditions as above.

V Weight increase in i percent Storage time, hours solution SolutionFrom this table follows that B is unchanged even afte more than' l00hours. 7 I. The'processing and particularly the spinning of suchsolutions to filaments is the easier and the more economic Lthe higherthe polymer content of the solution is. The

viscosity of the solution depends on the concentration and on themolecular weight of the polymer. Products :with an'average molecularweight exceeding 90,000 have in 20% solution at 20 C. such a highviscosity that they can'be moved at a useful speed only by the USCLOfconsiderable pressure. q

. Products having an average molecular weight in the range of 10,000 to90,000 are particularly suitable for the invention. Such products arereadily worked up with an about 60% nitric acid already at 0-50 C. to 10to .20% solutions, which have a viscosity of 20 to 500 poises :and areeasy to handle. By means of continuous mixing 'devices, for instancescrew-mixers, which simultaneously carry out the dissolving-and thepumping'of .the solutions, solutions of much higher concentration can be4 perature of 70 C. Thereto 1000 g. of acrylonitrile and 1.2 g. ofacetylperoxide are. added. After 2.4 and 6 hours 500 g. of acrylonitrileand 0.6 g. of acetylperoxide are admixed, so that a totality of 2500 g.of acrylonitrile get into the polymerisation vessel. After 9 hours inall the benzene'and the unchanged acrylonitrile are dis tilled 01f Thecontent of the autoclave now consists of a suspension of the obtainedacrylonitrile copolymer in water. The polymer is sucked ofi, washed anddried. 2000 to 2100 g. of a finely powdered polymer are obtained, itsapparent density is 0.40-.50, its molecular weight 57,000 to 59,000.

100 g. of a polymer produced in the above-mentioned way having amolecular weight of 57,3007are stirred at l015 C. in 400 g. of nitricacid of 53% by weight to a suspension and degassed in vacuo. Then 450 g.of HNO of 65 by weight'and 1 g. of urea are added and stirring iscontinued at C. under an absolute pressure of a mercury stem of -30 mm.Then the vacuum is released. An approximately water-clear solution ofpolyacrylonitrile in 59% nitric acid is obtained. The solution is storedat 15 C.

The appearance of the solution remains unchanged for more than 100hours. Only after 160 hours the formation of small gas bubbles isnoticeable. After 100 hours.

the polymer is precipitated from a sample of thG'SOhl! at 50 C. to theweight constancy. The precipitated product is soluble indimethylformamide. The defrjminationof the molecular weight and of thenitro en content gives 57,000 respectively 25.68%. Compare Kern andFernow, Journal fiir praktische'Chemiep. 287 (1942), and Ch. D. CoxeinGerman patent application No. 31,878 of 'Du Pont de Nemours & Co., page-S, "wherein a nitrogen content of 25.66% resp. 25.6% is indicated ascharacteristic .for polyacrylonitrile. A further examination of theconstancy" of the polymer innitric acid solution wascarn'ed out in thefollowing manner: A definite polymer quantity is weighted withanalytical exactness, dissolved in 58%"nitric,.acid at'20 Jre'sp. C. andaged in the thegm atorat 20 resp. 30? C. At definiteintewals," thesample was precipitated with water, washed and dried till the weight wasconstant. p The following weight increases are noted:

at20 I at 30? 8 hours 16 ho r 48 hours 96 hour:

no no. a

no-.- Decomposition).

Do. i v

; A further examination on the durability of the polymer in nitric acidsolution is carried out by the determination -of the water-absorption ofa film produced from the solution. For this purpose a specimen of theabove described solution is smeared on a glass slide and dried under airsuction in a drying'oven preheated to 120 C. leaving state, released andcrimped. These goperations are not- :an-object of the invention. 'In'spinning, the solvent may be washed out or removed by means of. hotgases. 1 The following examples will illustrate theidea of the Example.1

In a stainless steel stirring vessel resistant to pressure 4000 g.ofwater, 10 g. of a coplymer from 60 parts of vinylaeetate and parts ofmaleic acid anhydride, 10 g. :oficalciflmnitrate, 2 g. of nitricacid,120 g. of acetaldehyde' an'd 500g. of benzene are at a tem--the:door ajar. The dry film is stripped ofi from the .glass' slide inthe'waterbath and dried for 12 hours'at C; Afterwards the film isweighted accurately, then placed in water for 24 hours, superficiallydried with filter paper and finally -stored in a desiccator over water'for' 24 hours. The water-absorption is 2.1%. The waterabsorption ofacrylonitrile is approximately 2.5% according to I. B. Quig, Papers ofthe American Association of TextileTechnoldgists, March 1949, page 63.

These experiments show that .polyacrylonitrile ca'nbe dissolved innitric acid and thatyafter aging for some from this solution.

, Example 2 p 100 g. of polyacrylonitrile having an average molecularweight of 39,000, produced according't'o the abovementioned patentapplication Serial No. 236,081, now

abandoned, are mixed with 350 g. of 53% nitric acid at 20 C. An easilyflowing suspension is obtained. This suspension is placed under areduced pressure of about 20 mm. Hg and degassed, which requires aboutmin. Hereby, the polymer is not, or is onlyto a small degree, dissolved.Then, 350 .g. of 65% nitric acid and 1 g. of urea are admixed to thesuspension. Within a few minutes, a clear, lirnpid solution is obtained,which has at 20 C. a viscosity of- 34 poises. This solution is spunthrough a spinneret having holes of 0.1 mm. at a rate of 9 m./min. intoa water bath. The filament travels through the Water bath for a lengthof 8 cm., whereby only part of the nitric acid is removed; the filamentis then passed through a chamber of 3 m. length, where it is dried byhot air of 150 C. in order to drive off the rest of the nitric acid.After leaving the drying chamber, the filament is wound up at a speed ofm./min. and subsequently stretched at 170 C. to six times its length.Finally, the filament is again rinsed with water containing some ammoniato remove residual traces of acid. The filament shows the followingproperties: Dry tenacity 3.2 g./den., wet tenacity 3.3 g./den.,

loop tenacity 2.4 g./den., dry elongation 15%, wet elon- Example 3 100g. of a conjoint polymer consisting of 90 parts 02-? acrylonitrile and10 parts of vinylacetate, having a ...rr.-olecular weight of 30,000 andproduced according to "he above-mentioned patent application Serial No.294,-

.482, now Patent No. 2,671,768, are suspended in 250 parts ofnitromethane, degassed at reduced pressure and then dissolved by adding150 parts of nitric acid of 58% concentration. All these manipulationsare carried out at room temperature. Within a few minutes a water-clearsolution is obtained, which can be readily spun to filaments by aconventional dry spinning process. By pouring out the solution on aglass slide and evaporation of the solvent in an air current of 80-110C., a crystalclear film will be obtained which consists ofacrylonitrilevinyl acetate copolymer. The water absorption of the filmin water of C. in 24 hours is 2.1%, it is soluble in dimethyl-formamide.

Example 4 1000 g. of an acrylonitrile copolymer of the molecular weightof 60,000, produced in the manner as indicated in Example 1, areprocessed, with 10 g. of urea and 9000 g. of nitric acid of 59% byweight, to a solution as described in Example 1. The solution isfiltered through an acidproof steel wire gauze and in a manner knownspun by means of a spinning pump and a spinneret having 10 holes of 0.2mm diameter into a waterbath. Solution and coagulating bath are kept atl0-20 C. After leaving the bath, the filaments are washed with Wateruntil they are free from acids, then travel through a waterbath of 90 C.The bath contains 0.2% polyvinylalcohol, 2% glycerine and 0.1%phosphoric acid. The filaments are then dried at 100-120 C., stretchedat 140-150 C., and again shrunk at the same temperature by l520%. Thespinning pump and spinning speed is so regulated, that a filament formof 30 den. results.

The spinning experiment is kept going with the same solution for 96hours. The nitrogen content of the filament is during the wholeexperimental time about 25.6%.

The tensile strength of the filament is 4 3., the loop filament suppliessubstantially the same values in. the

. wet state.

. What I claim is:

copolymerizable unsaturated compounds other than acrylonitrile, attemperatures between 0 and 'C. in nitric acid having a. concentration of49 to 68 percent by weight to a stable Solution suitable for extrusion,:eli- .minating the nitric oxides during the dissolvingprocess,

and extruding said solution into a coagulating medium,

before a chemical modification of the dissolved acrylonitrile polymeroccurs.

2. The process of preparing shaped polyacrylonitrile containingstructures which comprises dissolving an acrylonitrile polymercontaining at least 80 percent of acrylonitrile in the polymer molecule,the balance being copolymerizable unsaturated compounds other thanacrylonitrile, at temperatures between 0 and 80 C. in nitric acid havinga concentration of 49 to 68 percent by weight to a stable solutionsuitable for extrusion, eliminating the nitric oxides by sucking themoff in vacuo during the dissolving process, and extruding said solutioninto a coagulating medium, before a chemical modifica tion of thedissolved acrylonitrile polymer occurs.

3. The process ofpreparing shaped polyacrylonitrile containingstructures which comprises dissolving an acrylonitrile polymer attemperatures between 0 and 80 I C. in nitric acid having a concentrationof 49 to 68 percent by weight to a stable solution suitable forextrusion, removing nitric oxides, and forming said solution into apredetermined shape, before a chemical modification of the dissolvedacrylonitrile polymer occurs, said acrylonitrile polymer containing atleast 80 percent by weight of polyacrylonitrile obtained by thecatalytic polymerization of acrylonitrile in the presence of a peroxideof an aliphatic acid, the balance being copolymerizable unsaturatedcompounds other than acrylonitrile.

4. The process of preparing shaped polyacrylonitrile forming saidsolution into a predetermined shape, before a chemical modification ofthe dissolved acrylonitrile polymer occurs.

5. The process as claimed inclaim 4 including the step of degassing thesuspension of the acrylonitrile containing polymer in nitric acid atreduced pressure prior to the heating step.

6. The process of preparing a solution' of a polymer of acrylonitrilewith nitric acid and water, comprising the steps of dispersing a polymerof acrylonitrile containing at least 80 percentof acrylonitrile, the:balance being copolymerizable unsaturated compounds other thanacrylonitrile, in a solution consisting of 46-54 percent nitric acid and54-46 percent of water at a temperature of 0-20 C., degassing thedispersion in vacuo,-

adding to said dispersion without raising the temperature concentratednitric acid until the HNO concentration of the dilute nitric acidreaches about 59%, and dis solving said polymer in said solution.

7. The process of preparing shaped polyacr'ylonitrile1 containingstructures, comprising the steps of dispersing a polymer ofacrylonitrile containing at least 80 percent.

otacrylonitrile, the balance being copolyrnerizable -unsaturatedcompounds other than acrylonitrile, in a solution consisting of 46-54percent of nitric-acid and dispersion in vacuo, addingt'o saiddispersion without ,raising thetemperatureconcentrated nitric acid untilthe unitricadd contains about59 percent of H NO dissolving 54-46 percentof water at a temperatureof 0-20 C., V

degassing -the dispersion in vacuo, heating said dispersion to atemperature not exceeding 80 so as to dissolverapidly the minuteparticles of the acrylonitrile polymerin said solution, and extrudingsaid solution into a coagulating bath before a chemical modification ofthe the; minute polymer; particles in-said nitric acid, and

extruding the solution into a coagulating bath consisting of water andless than 30 percent of nitric acid, before a chemical modification ofthe dissolved acrylonitrile polymer-occurs. 7

References Cited in the file of this patent UNITED STATES PATENTS2,579,451 Polson Dec. 18, 1951 2,671,768 Halbig Mar. 9, 1954 e FOREIGNPATENTS 665,004 Great Britain Ian. 16, 1952

8. A PROCESS FOR OBTAINING SHAPED ARTICLES WHICH COM PRISES THE STEPS OFDISPERSING AT A TEMPERATURE OF 0-20* C. A POLYMER OF ACRYLONITRILECONTAINING AT LEAST 80 PERCENT OF ACRYLONITRILE, THE BALANCE BEINGCOPOLYMERIZABLE UNSATURATED COMPOUNDS OTHER THAN ACRYLONITRILE, INNITRIC ACID OF 46-54 PERCENT HNO3 CONTENT, DEGASSING SAID DISPERSION INVACUO, ADDING TO SAID DISPERSION WITHOUT RAISING THE TEMPERATURECONCENTRATED NITRIC ACID UNTIL THE NITRIC ACID CONTAINS ABOUT 59 PERCENTOF HNO3, DISSOLVING THE MINUTE POLYMER PARTICLES IN SAID NITRIC ACID,AND EXTRUDING THE SOLUTION INTO A COAGULATING BATH CONSISTING OF WATERAND LESS THAN 30 PERCENT OF NITRIC ACID, BEFORE A CHEMICAL MODIFICATIONOF THE DISSOLVED ACRYLONTRILE POLYMER OCCURS.